Power meters are used to determine the power consumption of electric devices and systems in the industry and in the home. Various principles are known, e.g., the principle of the electromechanical Ferraris meter based on measuring the rotation of a disk driven by current- and/or voltage-proportional fields.
Modern power meters operate fully electronically. In many cases the current is detected on the inductive principle, whereby output signals of inductive current and voltage transformers are processed digitally and may then be made available for determining consumption and then for remote readings.
Electronic power meters using inductive current transformers are increasingly being used in the home. The low cost of manufacturing such meters to some extent plays an even greater role than their technical superiority. This necessitates the development of especially economical manufacturing methods for such current transformers. The load currents to be measured are in the range between a few mA and 100 A or more; this requires an accurate and calibratable energy measurement with a corresponding low phase error and amplitude error of the measurement signal in comparison with the primary current to be measured. In addition to the required accuracy, the cost of materials for such current transformers and thus the cost of the transformer core material in particular are also important in large-scale manufacturing.
In general, the following equation holds for the phase error of a current transformer
                              tan          ⁢                                          ⁢          φ                ≈                                                                              R                  Cu                                +                                  R                  B                                                            ω                ·                L                                      ·            cos                    ⁢                                          ⁢          δ                                    (        1        )            RB=resistance of the load;RCu=resistance of the secondary windingδ=loss angle of the transformer materialL=inductance of the secondary side of the current transformer.
The amplitude error is given by the equation
                              F          ⁡                      (            I            )                          ≈                                            -                                                                    R                    Cu                                    +                                      R                    B                                                                    ω                  ·                  L                                                      ·            sin                    ⁢                                          ⁢          δ                                    (        2        )            
The inductance L is defined as
                    L        =                              N            2            2                    ·                      μ            ′                    ·                      μ            0                    ·                                    A              Fe                                      l              Fe                                                          (        3        )            N2=secondary winding numberμ′=permeability of the transformer material (real component)μ0=general permeability constantAFe=iron cross section of the coreLFe=average path length of the iron of the core.
There is therefore a demand for cores having the highest possible permeability for implementation of current transformers that have a smaller volume and are therefore less expensive but still have a high precision.
To detect high currents, the transformer core requires a large inside diameter, which leads to a small ratio of the core outside diameter Da to the core inside diameter Di of usually <1.5 or even <1.25 with a small iron cross section AFe. However, such small diameter ratios lead to a high mechanical instability of the core and make it sensitive to any type of mechanical manipulation.
For these reasons, highly permeable materials such as ferrites or Permalloy materials have been used in the past as materials for such current transformer cores. However, ferrites have the disadvantage that their permeability is comparatively low and depends relatively greatly on temperature. One property of Permalloy materials is that although a low-phase error is achieved, it varies greatly with the current to be measured and/or the control of the magnetic core. Equalization of this variation is possible by suitable electronic wiring of the transformer or digital reprocessing of the measured values, but this constitutes an additional cost-intensive expense. Because of the fracture sensitivity of ferrites and the high magnetostriction and low saturation induction of both classes of materials, transformer cores having a small iron cross section that saves on material, i.e., a low Da/Di diameter ratio cannot be implemented.
Use of highly permeable magnetic cores made of nanocrystalline materials having a high saturation induction is also known from the state of the art, e.g., EP 05 04674 B1. However, these materials have a flat hysteresis loop in contrast with the present invention. Therefore, there is a demand for dimensioning current transformer cores having a large AFe with the permeability values that can be achieved in this way (μ approx. 60,000 to 120,000). Despite the good properties, especially with regard to the phase trend, economical use in mass production is therefore impossible.